Assembly layer for flexible display

ABSTRACT

The present invention is an assembly layer for a flexible device. Within a temperature range of between about −30° C. to about 90° C., the assembly layer has a shear storage modulus at a frequency of 1 Hz that does not exceed about 2 MPa, a shear creep compliance (J) of at least about 6×10 −6    1 /Pa measured at 5 seconds with an applied shear stress between about 50 kPa and about 500 kPa, and a strain recovery of at least about 50% at at least one point of applied shear stress within the range of about 5 kPa to about 500 kPa within about 1 minute after removing the applied shear stress. The assembly layer includes at least one of a polyisoprene, a polybutadiene, an olefin block copolymer, a polyisobutylene, and high alkyl polyolefin.

FIELD OF THE INVENTION

The present invention is related generally to the field of assemblylayers. In particular, the present invention is related to flexibleassembly layers used in flexible devices, such as flexible electronicdisplays and flexible photovoltaic materials.

BACKGROUND

A common application of pressure-sensitive adhesives in the industrytoday is in the manufacturing of various displays, such as computermonitors, TVs, cell phones and small displays (in cars, appliances,wearables, electronic equipment, etc.). Flexible electronic displays,where the display can be bent freely without cracking or breaking, is arapidly emerging technology area for making electronic devices using,for example, flexible plastic substrates. This technology allowsintegration of electronic functionality into non-planar objects,conformity to desired design, and flexibility during use that can giverise to a multitude of new applications.

With the emergence of flexible electronic displays, there is anincreasing demand for adhesives, and particularly for optically clearadhesives (OCA), to serve as an assembly layer or gap filling layerbetween an outer cover lens or sheet (based on glass, PET, PC, PMMA,polyimide, PEN, cyclic olefin copolymer, etc.) and an underlying displaymodule of electronic display assemblies. The presence of the OCAimproves the performance of the display by increasing brightness andcontrast, while also providing structural support to the assembly. In aflexible assembly, the OCA will also serve at the assembly layer, whichin addition to the typical OCA functions, may also absorb most of thefolding induced stress to prevent damage to the fragile components ofthe display panel and protect the electronic components from breakingunder the stress of folding. The OCA layer may also be used to positionand retain the neutral bending axis at or at least near the fragilecomponents of the display, such as for example the barrier layers, thedriving electrodes, or the thin film transistors of an organic lightemitting display (OLED).

If used outside of the viewing area of a display or photo-active area ofa photovoltaic assembly, it is not necessary that the flexible assemblylayer is optically clear. Indeed, such material may still be useful forexample as a sealant at the periphery of the assembly to allow movementof the substrates while maintaining sufficient adhesion to seal thedevice.

Typical OCAs are visco-elastic in nature and are meant to providedurability under a range of environmental exposure conditions and highfrequency loading. In such cases, a high level of adhesion and somebalance of visco-elastic property is maintained to achieve goodpressure-sensitive behavior and incorporate damping properties in theOCA. However, these properties are not fully sufficient to enablefoldable or durable displays.

Due to the significantly different mechanical requirements for flexibledisplay assemblies, there is a need to develop novel adhesives forapplication in this new technology area. Along with conventionalperformance attributes, such as optical clarity, adhesion, anddurability, these OCAs need to meet a new challenging set ofrequirements such as bendability and recoverability without defects anddelamination.

SUMMARY

In one embodiment, the present invention is an assembly layer for aflexible device. Within a temperature range of between about −30° C. toabout 90° C., the assembly layer has a shear storage modulus at afrequency of 1 Hz that does not exceed about 2 MPa, a shear creepcompliance (J) of at least about 6×10⁻⁶ 1/Pa measured at 5 seconds withan applied shear stress between about 50 kPa and about 500 kPa, and astrain recovery of at least about 50% at at least one point of appliedshear stress within the range of about 5 kPa to about 500 kPa withinabout 1 minute after removing the applied shear stress. The assemblylayer includes at least one of a polyisoprene, a polybutadiene, anolefin block copolymer, a polyisobutylene, and high alkyl polyolefin.

In another embodiment, the present invention is a laminate including afirst flexible substrate, a second flexible substrate, and an assemblylayer positioned between and in contact with the first flexiblesubstrate and the second flexible substrate. Within a temperature rangeof between about −30° C. to about 90° C., the assembly layer has a shearstorage modulus at a frequency of 1 Hz that does not exceed about 2 MPa,a shear creep compliance (J) of at least about 6×10⁻⁶ 1/Pa measured at 5seconds with an applied shear stress between about 50 kPa and about 500kPa, and a strain recovery of at least about 50% at at least one pointof applied shear stress within the range of about 5 kPa to about 500 kPawithin about 1 minute after removing the applied shear stress. Theassembly layer includes at least one of a polyisoprene, a polybutadiene,an olefin block copolymer, a polyisobutylene, and high alkyl polyolefin.

In yet another embodiment, the present invention is a method of adheringa first substrate and a second substrate, wherein both of the first andthe second substrates are flexible. The method includes positioning anassembly layer between the first substrate and the second substrate andapplying pressure and/or heat to form a laminate. Within a temperaturerange of between about −30° C. to about 90° C., the assembly layer has ashear storage modulus at a frequency of 1 Hz that does not exceed about2 MPa, a shear creep compliance (J) of at least about 6×10⁻⁶ 1/Pameasured at 5 seconds with an applied shear stress within about 50 kPaand about 500 kPa, and a strain recovery of at least about 50% at atleast one point of applied shear stress within the range of about 5 kPato about 500 kPa within about 1 minute after removing the applied shearstress. The assembly layer includes at least one of a polyisoprene, apolybutadiene, an olefin block copolymer, a polyisobutylene, and highalkyl polyolefin.

DETAILED DESCRIPTION

The present invention is an assembly layer usable, for example, in aflexible devices, such as electronic displays, flexible photovoltaiccells or solar panels, and wearable electronics. As used herein, theterm “assembly layer” refers to a layer that possesses the followingproperties: (1) adherence to at least two flexible substrates and (2)sufficient ability to hold onto the adherends during repeated flexing topass the durability testing. As used herein, a “flexible device” isdefined as a device that can undergo repeated flexing or roll up actionwith a bend radius as low as 200 mm, 100 mm, 50 mm, 20 mm, 10 mm, 5 mm,or even less than 2 mm. The assembly layer is soft, is predominantlyelastic with good adhesion to plastic films or other flexible substrateslike glass, and has high tolerance for shear loading. In addition, theassembly layer has relatively low modulus, high percent compliance atmoderate stress, a low glass transition temperature, generation ofminimal peak stress during folding, and good strain recovery afterapplying and removing stress, making it suitable for use in a flexibleassembly because of its ability to withstand repeated folding andunfolding. Under repeated flexing or rolling of a multi-layeredconstruction, the shear loading on the adhesive layers becomes verysignificant and any form of stress can cause not only mechanical defects(delamination, buckling of one or more layers, cavitation bubbles in theadhesive, etc.) but also optical defects or Mura. Unlike traditionaladhesives that are mainly visco-elastic in character, the assembly layerof the present invention is predominantly elastic at use conditions, yetmaintains sufficient adhesion to pass a range of durabilityrequirements. In one embodiment, the assembly layer is optically clearand exhibits low haze, high visible light transparency, anti-whiteningbehavior, and environmental durability.

The assembly layer of the present invention is prepared from selectcompositions and cross-linked at different levels to offer a range ofelastic properties, while still generally meeting all optically clearrequirements. For example, an assembly layer used within a laminate witha folding radius as low as 5 mm or less can be obtained without causingdelamination or buckling of the laminate or bubbling of the adhesive.Examples of suitable assembly layers compositions include at least oneof, but are not limited to: an acrylic, an acrylic block copolymer, aphysically cross-linked silicone elastomer, a covalently cross-linkedsilicone elastomer, an ionomerically cross-linked silicone elastomerreagent forming mixture, a polyurethane, a covalently crosslinkedpolyurethane comprising silicone-based moieties, a polyisoprene, apolybutadiene, an olefin block copolymer, a polyisobutylene, and a highalkyl polyolefin.

In one embodiment, the assembly layer composition is acrylic-based andis derived from precursors that include at least one alkyl(meth)acrylateester having between about 1 to about 24 carbon atoms in the alkyl groupand a free-radical generating initiator. In some embodiments, theprecursor mixture includes a polar copolymerizable monomer and across-linker.

In one embodiment, the assembly layer composition is acrylic blockcopolymer-based. As used herein, the term “acrylic” is synonymous with“(meth)acrylate” and refers to polymeric material that is prepared fromacrylates, methacrylates, or derivatives thereof. As used herein, theterm “polymer” refers to a polymeric material that is a homopolymer or acopolymer. As used herein, the term “homopolymer” refers to a polymericmaterial that is the reaction product of one monomer. As used herein,the term “copolymer” refers to a polymeric material that is the reactionproduct of at least two different monomers. As used herein, the term“block copolymer” refers to a copolymer formed by covalently bonding atleast two different polymeric blocks to each other, but that does nothave a comb-like structure. The two different polymeric blocks arereferred to as the A block and the B block.

In one embodiment, the assembly layer of the present invention includesat least one multi-block copolymer (for example, ABA or star block (AB)nwhere n represents the number of arms in the star block) and an optionaldiblock (AB) copolymer. Such block copolymers are physicallycross-linked due to the phase separation of a hard A block and a soft Bblock. Additional cross-linking may be introduced by a covalentcrosslinking mechanism (i.e. thermally induced or using UV irradiation,high energy irradiation such as e-beam, or ionic crosslinking). Thisadditional cross-linking can be done in the hard block A, the soft blockB, or both. In another embodiment, the acrylic block copolymer assemblylayer is based on at least one multi-block copolymer, having, forexample, a poly methyl methacrylate (PMMA) hard A blocks and one or morepoly-n-butyl acrylate (PnBA) soft B blocks. In yet another embodiment,the acrylic block copolymer-based assembly layer is based on at leastone multi-block copolymer, having, for example, a polymethylmethacrylate (PMMA) hard A blocks and one or more poly-n-butyl acrylate(PnBA) soft B blocks, combined with at least one AB diblock copolymer,having, for example, a poly methyl methacrylate (PMMA) hard A block anda poly-n-butyl acrylate (PnBA) soft B block.

The assembly layer contains a block copolymer that includes the reactionproduct of at least two A block polymeric units and at least one B blockpolymeric unit (i.e., at least two A block polymeric units arecovalently bonded to at least one B block polymeric unit). Each A block,which has a Tg of at least 50° C., is the reaction product of a firstmonomer composition that contains an alkyl methacrylate, an aralkylmethacrylate, an aryl methacrylate, or a combination thereof. The Ablock may also be made from styrenic monomers, such as styrene. The Bblock, which has a Tg no greater than about 10° C., particularly nogreater than about 0° C., and more particularly no greater than about−10° C., is the reaction product of a second monomer composition thatcontains an alkyl (meth)acrylate, a heteroalkyl (meth)acrylate, a vinylester, or a combination thereof. The block copolymer contains betweenabout 5 and about 50 weight percent A block and between about 50 toabout 95 weight percent B block based on the weight of the blockcopolymer.

In one embodiment, the assembly layer composition is silicone-based andincludes at least one of physically cross-linked silicone elastomers,covalently cross-linked silicone elastomers and ionomericallycross-linked silicone elastomers. Suitable silicone elastomeric polymersinclude for example, urea-based silicone copolymers, oxamide-basedsilicone copolymers, amide-based silicone copolymers, urethane-basedsilicone copolymers, and mixtures thereof. Suitable covalentlycross-linked silicones include those derived from silicone elastomerforming reagents that undergo for example condensation curing, additioncuring, and thiol-ene type reaction. The term “silicone-based” as usedherein refers to macromolecules (e.g., polymer of copolymer) thatcontain a majority of silicone units. The terms silicone or siloxane areused interchangeably and refer to units with a siloxane (—Si(R₁)₂O—)repeating units where R₁ is defined below. In many embodiments, R₁ is analkyl. In some embodiments, the assembly layer includes a MQ resin. Forurethane-based silicone copolymers, crosslinking may be achieved byaddition curing, for example by photo-crosslinking of (meth)acrylatefunctional groups comprising the urethane-based silicone copolymers.Also, for urethane-based silicone copolymers, crosslinking may beachieved by condensation curing, for example by reaction of carboxylicacid groups comprising the urethane-based silicone copolymers curingwith polyaziridines or carbodiimides. Multi-valent metal ions, such asthose based on zinc, aluminum, titanium, and the like may also be usedin combination with an acid group containing urethane-based siliconecopolymer to generate an ionomeric crosslinked network. Theurethane-based silicone copolymers may also contain ionic functionalgroups such as sulfonate groups (—SO₃-M+), wherein M is Na or Li. Theurethane-based silicone copolymers may also contain ionic functionalgroups such as quaternary ammonium salts (—N(R₅)₃+X—), wherein R₅ isindependently alkyl, aryl, aralkyl, alkaryl, alkylene, arylene,aralkylene, or alkarylene, and X— is a counterion. The counterion may befor example a halide (Cl—, Br—, etc.) or a sulfonate (R₆SO₃—) wherein R₆is alkyl, aryl, aralkyl, or alkaryl. These urethane-based siliconecopolymers may be prepared using diisocyanates, carbinol cappedsilicone-based polyols, and other polyols. These polyurethanes maycomprise units of the following formulas:

wherein R_(ISO) is the residue of a diisocyanate and R_(DIOL) is theresidue of a diol,

-   -   wherein R_(DIOL) comprises units of the formula,    -   a) —R₂—,        -   wherein R₂ is a straight, branched chain, or cyclic alkylene            or oxyalkylene,    -   b) silicone-based units of the formula        -Q-(R₁)₂SiO—(Si(R₁)₂O)_(n)—Si(R₁)₂-Q-,        -   wherein R₁ is independently alkyl or aryl and wherein Q is a            connecting group of valency 2 or greater, and n=5-200,        -   and at least one of the units c), d), e), and f) wherein    -   c) acrylate containing units of the formula        —R₂-(A)_(b)-Q-(A)_(b)-R₂—,        -   and A is a (meth)acryl functional group X—C(O)C(R₄)═CH₂,            -   wherein X is selected from O—, or NR₃,            -   wherein R₃ is H or alkyl of 1-4 carbon atoms,            -   wherein R₄ is alkyl of 1-4 carbon atoms, and            -   wherein b is 1-3, and Q and R₂ are defined as above,    -   d) carboxylic acid containing units of the formula        —R₂-Q-(CO₂H)_(b)—R₂—, wherein        -   —R₂—, b, and Q, are defined as above and    -   e) sulfonate acid salt containing units of the formula        —R₂-Q-(SO₃M)_(b)-R₂—, wherein        -   —R₂—, b, and Q, are defined as above and M is Na or Li,    -   f) quaternary ammonium salts units of the formula        —R₂-Q-(N(R₅)₃+X—)_(b)—R₂—, wherein —R₂—, b, and Q, are defined        as above and R₅ is independently alkyl, aryl, aralkyl, alkaryl,        alkylene, arylene, aralkylene, or alkarylene, and X— is a        counterion.

The n in formula -Q-(R₁)₂SiO—(Si(R₁)₂O)_(n)—Si(R₁)₂-Q-is 5-200,particularly 10-100, and more particularly 15-75.

Q can be a straight or branched chain or cycle-containing connectinggroup. Q can include a covalent bond, an alkylene, an arylene, anaralkylene, an alkarylene. Q can optionally include heteroatoms such asO, N, and S, and combinations thereof. Q can also optionally include aheteroatom-containing functional groups such as hydroxyl, carbonyl orsulfonyl, and combinations thereof.

Examples of diisocyanates include isophorone diisocyanate1,6-hexamethylene diisocyanate, 4,4′-methylenebis (cyclohexylisocyanate), mixtures of 2,2,4-trimethyl-1,6-hexamethylene diiisocyanateand 2,4,4-trimethyl-1,6-hexamethylene diiisocyanate available fromEvonik as Vestanat TMDI. Small amounts of polyisocyanates withfunctionality of greater that 2 may also be used. Many of themultifunctional isocyanates of greater than 2 functionality exist as adistribution of materials. For example, hexamethylene diisocyanate basedisocyanate oligomers such as biuret multi-isocyanates, which areavailable under the trade designation DESMODUR N100, exist as a mixtureof hexamethylene diisocyanate, hexamethylene diisocyanate biurettrimers, hexamethylene diisocyanate biuret pentamers, hexamethylenediisocyanate biuret heptamers, and so on. The same is true forhexamethylene diisocyanate based isocyanurate multi-isocyanatesavailable under the trade designation DESMODUR N3300. Biuret andisocyanurate multi-isocyanates may be based on other diisocyanates suchas isophorone diisocyanate, or tolylene diisocyanate.

Examples of the diols include 1,2-ethylene glycol, 1,4-butanediol, andthe like. Additionally, a small amount of the diols may be replaced withhydrazine, or diamines. Silicone-based diols include Dow Corningdimethylsiloxane dicarbinol 5562, and Gelest dimethylsiloxanedicarbinol, DMS-C21. Acrylated diols include Epoxy Acrylate diol DenacolDA-920, polypropylene glycol (3) diglycidyl ether reacted with two molesof acrylic acid. Acid functional diols include 2,2-dimethylol-propanoicacid. Small amounts of polyols of functionality greater than 2 such asglycerol and polycaprolactone triols may be used.

In one embodiment, the assembly layer composition is based on apolyurethane. Such polyurethanes may be derived from a polyester polyolor polyetherpolyol reacted with a multifunctional isocyanate. In view ofthe lower modulus and lower Tg, polyether polyol based polyurethanes maybe preferred, although some copolymerization with a polyester polyol ispossible to tune the mechanical properties of the polymer. Thepolyurethanes may be sufficiently tacky to adhere reliably to theflexible substrates, but their adhesion and modulus can be furtherenhanced by using tackifiers and plasticizers as part of theformulation. These tackifiers and plasticizers are typically reducingthe modulus of the polyurethane and they do not interfere with thephysical crosslinking provided by the hard segments in the polyurethanepolymer. If desired, the polyurethanes can also be covalentlycrosslinked, such as for example using polyurethanes with acrylatefunctionality, silane functionality, and the like.

In one embodiment the assembly layer composition is based on a diene,such as a polyisoprene or polybutadiene. Examples of such dienes includeliquid isoprene and butadiene diol or isoprene diol. Liquid isoprene maybe crosslinked using multifunctional thiol reagents which in thepresence of a catalyst and heat or actinic irradiation will yield apredominantly elastic assembly layer material. The butadiene diol orisoprene diol can be used as the main polyol in a polyurethane polymeror they may be acrylate functionalized to allow for free-radical inducedcrosslinking or copolymerization with other acrylates or methacrylatesto yield the assembly layer. If desired the modulus and adhesion of thediene based assembly layer can be adjusted by using tackifiers and/orplasticizers as part of the assembly layer formulation.

In one embodiment the assembly layer is based on an olefin polymer, suchas an olefin block copolymer, a polyisobutylene, and a high alkylpolyolefin. An example of an olefin blockcopolymer is Infuse™ availablefrom Dow Chemical, Midland, Mich. Such blockcopolymer have alternatingblocks of semi-crystalline, reinforcing segments and soft, elastomericsegments. Polyisobutylenes, such as Vistanex™ available from ExxonChemical, Baton Rouge, La. are available in a range of molecularweights. Typically blends of high and low molecular polyisobutylenes areused, where the high molecular weight polymer provides the elastomericproperties and the low molecular weight polymers provide the moreviscous properties and enhance the tackiness of the composition. Highalkyl olefins include homopolymers and copolymers of high alkyl olefinmonomers. High alkyl olefins include for example hexane and octene. Ifdesired, they can be copolymerized with a smaller fraction of low alkylolefin monomers, such as ethylene and propylene. Too high a content oflow alkyl olefin may reduce adhesion and increase the modulus too much.Some of these polymers used metallocene type catalysts to carry out thepolymerization. If desired, the olefin based assembly layer cancovalently crosslinked and compounded with tackifiers to adjust modulusand tackiness. Typical tackifiers include hydrocarbon (i.e. known as C5,C9, or cyclopentadiene based) tackifiers, which can be hydrogenated. Insome cases, the olefin may also be cohesively reinforced usinginter-penetrating polymer networks. An example of this can be found inU.S. Pat. No. 8,232,350 (Bharti et al.).

Other materials can be added to the assembly layer composition forspecial purposes, including, for example: thermal or photoinitiators,cross-linkers, tackifiers, molecular weight control agents, couplingagent, oils, plasticizers, antioxidants, UV stabilizers, UV absorbers,pigments, catalysts, curing agents, polymer additives, nanoparticles,and other additives. In cases where the assembly layer needs to beoptically clear, other materials can be added to the monomer mixture,provided that they do not significantly reduce the optical clarity ofthe assembly layer after polymerization and coating. As used herein, theterm “optically clear” refers to a material that has a luminoustransmission of greater than about 90 percent, a haze of less than about2 percent, and opacity of less than about 1 percent in the 400 to 700 nmwavelength range. Both the luminous transmission and the haze can bedetermined using, for example, ASTM-D 1003-92. Typically, the opticallyclear assembly layer is visually free of bubbles.

In one embodiment, the assembly layer may be substantially free of acidto eliminate indium tin oxide (ITO) and metal trace corrosion thatotherwise could damage touch sensors and their integrating circuits orconnectors. “Substantially free” as used in this specification meansless than about 2 parts by weight, particularly less than about 1 parts,and more particularly less than about 0.5 parts.

The assembly layer can be manufactured by any method known in the artdepending on the particular components of the assembly layer. In oneembodiment, the assembly layer components can be blended into aprecursor mixture. This mixture is then coated on a liner or directly ona substrate and completely polymerized by additional exposure to heat oractinic irradiation.

In another process, the assembly layer components can be blended with asolvent to form a mixture. The mixture can be polymerized by exposure toheat or actinic radiation (to decompose initiators in the mixture). Across-linker and additional additives such as tackifiers andplasticizers may be added to the solvated polymer which may then becoated on a liner and run through an oven to dry off solvent to yieldthe coated adhesive film. Solventless polymerization methods, such asthe continuous free radical polymerization method described in U.S. Pat.Nos. 4,619,979 and 4,843,134 (Kotnour et al.); the essentially adiabaticpolymerization methods using a batch reactor described in U.S. Pat. No.5,637,646 (Ellis); and the methods described for polymerizing packagedpre-adhesive compositions described in U.S. Pat. No. 5,804,610 (Hamer etal.) may also be utilized to prepare the polymers.

In another process, the polymer for the assembly layer may be preformed.In such case the polymer may be dissolved in a suitable organic solvent,additives such a tackifier, a plasticizer, or if needed a crosslinkermay be added. The solvated polymer mixture may then be coated on a linerand run through an oven to dry off solvent to yield the coated assemblylayer.

The assembly layer composition can be coated onto a release liner,coated directly onto the carrier film, co-extruded with a flexiblesubstrate film, or formed as a separate layer (e.g., coated onto arelease liner) and then laminated to the flexible substrate. In someembodiments, the assembly layer is disposed between two release linersfor subsequent lamination to the flexible substrate.

The disclosed compositions or precursor mixtures may be coated by anyvariety of techniques known to those of skill in the art, such as rollcoating, spray coating, knife coating, die coating, and the like.Alternatively, the precursor composition may also be delivered as aliquid to fill the gap between the two substrates and subsequently beexposed to heat or UV to polymerize and cure the composition in betweenthe two substrates.

The present invention also provides laminates including the assemblylayer. A laminate is defined as a multi-layer composite of at least oneassembly layer sandwiched between two flexible substrate layers ormultiples thereof. For example the composite can be a 3 layer compositeof substrate/assembly layer/substrate; a 5-layer composite ofsubstrate/assembly layer/substrate/assembly layer/substrate, and so on.The thickness, mechanical, electrical (such as dielectric constant), andoptical properties of each of the flexible assembly layers in suchmulti-layer stack may be the same but they can also be different inorder to better fit the design and performance characteristics of thefinal flexible device assembly. The laminates have at least one of thefollowing properties: optical transmissivity over a useful lifetime ofthe article in which it is used, the ability to maintain a sufficientbond strength between layers of the article in which it is used,resistance or avoidance of delamination, and resistance to bubbling overa useful lifetime. The resistance to bubble formation and retention ofoptical transmissivity can be evaluated using accelerated aging tests.In an accelerated aging test, the assembly layer is positioned betweentwo substrates. The resulting laminate is then exposed to elevatedtemperatures often combined with elevated humidity for a period of time.Even after exposure to elevated temperature and humidity, the laminate,including the assembly layer, will retain optical clarity. For example,the assembly layer and laminate remain optically clear after aging at70° C. and 90% relative humidity for approximately 72 hours andsubsequently cooling to room temperature. After aging, the averagetransmission of the adhesive between 400 nanometers (nm) and 700 nm isgreater than about 90% and the haze is less than about 5% andparticularly less than about 2%.

In use, the assembly layer will resist fatigue over thousands or more offolding cycles over a broad temperature range from well below freezing(i.e., −30 degrees C., −20 degrees C., or −10 degrees C.) to about 70,85 or even 90° C. In addition, because the display incorporating theassembly layer may be sitting static in the folded state for hours, theassembly layer has minimal to no creep, preventing significantdeformation of the display, deformation which may be only partiallyrecoverable, if at all. This permanent deformation of the assembly layeror the panel itself could lead to optical distortions or Mura, which isnot acceptable in the display industry. Thus, the assembly layer is ableto withstand considerable flexural stress induced by folding a displaydevice as well as tolerating high temperature, high humidity (HTHH)testing conditions. Most importantly, the assembly layer hasexceptionally low storage modulus and high elongation over a broadtemperature range (including well below freezing; thus, low glasstransition temperatures are preferred) and are cross-linked to producean elastomer with little or no creep under static load.

During a folding or unfolding event, it is expected that the assemblylayer will undergo significant deformation and cause stresses. Theforces resistant to these stresses will be in part determined by themodulus and thickness of the layers of the folding display, includingthe assembly layer. To ensure a low resistance to folding as well asadequate performance, generation of minimal stress and good dissipationof the stresses involved in a bending event, the silicone-based assemblylayer has a sufficiently low storage or elastic modulus, oftencharacterized as shear storage modulus (G′). To further ensure that thisbehavior remains consistent over the expected use temperature range ofsuch devices, there is minimal change in G′ over a broad and relevanttemperature range. In one embodiment, the relevant temperature range isbetween about −30° C. to about 90° C. In one embodiment, the shearmodulus is less than about 2 MPa, particularly less than about 1 MPa,more particularly less than about 0.5 MPa, and most particularly lessthan about 0.3 MPa over the entire relevant temperature range.Therefore, it is preferred to position the glass transition temperature(Tg), the temperature at which the material transitions to a glassystate, with a corresponding change in G′ to a value typically greaterthan about 10⁷ Pa, outside and below this relevant operating range. Inone embodiment, the Tg of the assembly layer in a flexible display isless than about 10° C., particularly less than about −10° C., and moreparticularly less than about −30° C. As used herein, the term “glasstransition temperature” or “Tg” refers to the temperature at which apolymeric material transitions from a glassy state (e.g., brittleness,stiffness, and rigidity) to a rubbery state (e.g., flexible andelastomeric). The Tg can be determined, for example, using a techniquesuch as Dynamic Mechanical Analysis (DMA). In one embodiment, the Tg ofthe acrylic-based assembly layer in a flexible display is less thanabout 10° C., particularly less than about −10° C., and moreparticularly less than about −30° C.

The assembly layer is typically coated at a dry thickness of less thanabout 300 microns, particularly less than about 50 microns, particularlyless than about 20 microns, more particularly less than about 10microns, and most particularly less than about 5 microns. The thicknessof the assembly layer may be optimized according to the position in theflexible display device. Reducing the thickness of the assembly layermay be preferred to decrease the overall thickness of the device as wellas to minimize buckling, creep, or delamination failure of the compositestructure.

The ability of the assembly layer to absorb the flexural stress andcomply with the radically changing geometry of a bend or fold can becharacterized by the ability of such a material to undergo high amountsof strain or elongation under relevant applied stresses. This compliantbehavior can be probed through a number of methods, including aconventional tensile elongation test as well as a shear creep test. Inone embodiment, in a shear creep test, the assembly layer exhibits ashear creep compliance (J) of at least about 6×10⁻⁶ 1/Pa, particularlyat least about 20×10⁻⁶ 1/Pa, about 50×10⁻⁶ 1/Pa, and more particularlyat least about 90×10⁻⁶ 1/Pa under an applied shear stress of betweenabout 5 kPa to about 500 kPa, particularly between about 20 kPa to about300 kPa, and more particularly between about 50 kPa to about 200 kPa.The test is normally conducted at room temperature but could also beconducted at any temperature relevant to the use of the flexible device.

The assembly layer also exhibits relatively low creep to avoid lastingdeformations in the multilayer composite of a display following repeatedfolding or bending events. Material creep may be measured through asimple creep experiment in which a constant shear stress is applied to amaterial for a given amount of time. Once the stress is removed, therecovery of the induced strain is observed. In one embodiment, the shearstrain recovery within 1 minute after removing the applied stress (at atleast one point of applied shear stress within the range of about 5 kPato about 500 kPa) at room temperature is at least about 50%,particularly at least about 60%, about 70% and about 80%, and moreparticularly at least about 90% of the peak strain observed at theapplication of the shear stress. The test is normally conducted at roomtemperature but could also be conducted at any temperature relevant tothe use of the flexible device.

Additionally, the ability of the assembly layer to generate minimalstress and dissipate stress during a fold or bending event is criticalto the ability of the assembly layer to avoid interlayer failure as wellas its ability to protect the more fragile components of the flexibledisplay assembly. Stress generation and dissipation may be measuredusing a traditional stress relaxation test in which a material is forcedto and then held at a relevant shear strain amount. The amount of shearstress is then observed over time as the material is held at this targetstrain. In one embodiment, following about 500% shear strain,particularly about 600%, about 700%, and about 800%, and moreparticularly about 900% strain, the amount of residual stress (measuredshear stress divided by peak shear stress) observed after 5 minutes isless than about 50%, particularly less than about 40%, about 30%, andabout 20%, and more particularly less than about 10% of the peak stress.The test is normally conducted at room temperature but could also beconducted at any temperature relevant to the use of the flexible device.

As an assembly layer, the assembly layer must adhere sufficiently wellto the adjacent layers within the display assembly to preventdelamination of the layers during the use of the device that includesrepeated bending and folding actions. While the exact layers of thecomposite will be device specific, adhesion to a standard substrate suchas PET may be used to gauge the general adhesive performance of theassembly layer in a traditional 180 degree peel test mode. The adhesivemay also need sufficiently high cohesive strength, which can bemeasured, for example, as a laminate of the assembly layer materialbetween two PET substrates in a traditional T-peel mode.

When the assembly layer is placed between two substrates to form alaminate and the laminate is folded or bent and held at a relevantradius of curvature, the laminate does not buckle or delaminate betweenall use temperatures (−30° C. to 90° C.), an event that would representa material failure in a flexible display device. In one embodiment, amultilayer laminate containing the assembly layer does not exhibitfailure when placed within a channel forcing a radius of curvature ofless than about 200 mm, less than about 100 mm, less than about 50 mm,particularly less than about 20 mm, about 15 mm, about 10 mm, and about5 mm, and more particularly less than about 2 mm over a period of about24 hours. Furthermore, when removed from the channel and allowed toreturn from the bent orientation to its previously flat orientation, alaminate including the assembly layer of the present invention does notexhibit lasting deformation and rather rapidly returns to a flat ornearly flat orientation. In one embodiment, when held for 24 hours andthen removed from the channel that holds the laminate with a radius ofcurvature of particularly less than about 50 mm, particularly less thanabout 20 mm, about 15 mm, about 10 mm, and about 5 mm, and moreparticularly less than about 3 mm, the composite returns to a nearlyflat orientation where the final angle between the laminate, thelaminate bend point and the return surface is less than about 50degrees, more particularly less than about 40 degrees, about 30 degrees,and about 20 degrees, and more particularly less than about 10 degreeswithin 1 hour after the removal of the laminate from the channel. Inother words, the included angle between the flat parts of the foldedlaminate goes from 0 degrees in the channel to an angle of at leastabout 130 degrees, particularly more than about 140 degrees, about 150degrees, and about 160 degrees, and more particularly more than about170 degrees within 1 hour after removal of the laminate from thechannel. This return is preferably obtained under normal usageconditions, including after exposure to durability testing conditions.

In addition to the static fold testing behavior described above, thelaminate including first and second substrates bonded with the assemblylayer does not exhibit failures such as buckling or delamination duringdynamic folding simulation tests. In one embodiment, the laminate doesnot exhibit a failure event between all use temperatures (−30° C. to 90°C.) over a dynamic folding test in free bend mode (i.e. no mandrel used)of greater than about 10,000 cycles, particularly greater than about20,000 cycles, about 40,000 cycles, about 60,000 cycles, and about80,000 cycles, and more particularly greater than about 100,000 cyclesof folding with a radius of curvature of less than about 50 mm,particularly less than about 20 mm, about 15 mm, about 10 mm, and about5 mm, and more particularly less than about 3 mm.

To form a flexible laminate, a first substrate is adhered to a secondsubstrate by positioning the assembly layer of the present inventionbetween the first substrate and the second substrate. Additional layersmay also be included to make a multi-layer stack. Pressure and/or heatis then applied to form the flexible laminate.

Examples

The present invention is more particularly described in the followingexamples that are intended as illustrations only, since numerousmodifications and variations within the scope of the present inventionwill be apparent to those skilled in the art. Unless otherwise noted,all parts, percentages, and ratios reported in the following example areon a weight basis.

Test Method 1. Dynamic Mechanical Analysis

Dynamic mechanical analysis was used to probe the modulus as a functionof temperature as well as to determine the glass transition temperature(T_(g)) of the material. An 8 mm diameter by about 1 mm thick disk ofthe assembly layer was placed between the probes of a DHR parallel platerheometer (TA Instruments, New Castle, Del.). A temperature scan wasperformed by ramping from −45° C. to 50° C. at 3° C./min. During thisramp, the samples was oscillated at a frequency of 1 Hz and a strain ofapproximately 0.4%. The shear storage modulus (G′) was recorded atselected key temperatures. The T_(g) of the material was also determinedas the peak in the tan delta vs. temperature profile. To ensuresufficient compliance of the assembly material over the typical range ofuse temperatures, it is preferred to have the shear storage modulusbelow about 2 MPa over the entire temperature range from about −20° C.to about 40° C. when measured using the test described above.

Test Method 2. Creep Test

The assembly layer samples were subjected to a creep test by placing a 8mm diameter by about 1.0 mm thick disk in a DHR parallel plate rheometerand applying a shear stress of 95 kPa for 5 seconds at which time theapplied stress was removed and the sample was allowed to recover in thefixtures for 60 seconds. The peak shear strain at 5 seconds and theamount of strain recovery after 60 seconds were recorded. The shearcreep compliance, J, at any time following the application of the stressis defined as the ratio of the shear strain at that time divided by theapplied stress. To ensure sufficient compliance within the assemblylayer, it is preferred that the peak shear strain after applying theload in the test described above is greater than about 200%.Furthermore, to minimize material creep within the flexible assembly, itis preferred that the material recover greater that about 50% strain 60seconds after the applied stress is removed. The percent recoverablestrain is defined as ((S₁−S₂)/S₁)*100 where S₁ is the shear strainrecorded at the peak at 5 seconds after applying the stress and S₂ isthe shear strain measured at 60 seconds after the applied stress isremoved.

TABLE 1 Materials Acronym Name Supplier B 50 SF Polyisobutylene BASFCorporation, Florham Park, NJ SR833 S Tricyclodecane trimethanolSartomer, diacrylate Exton, PA Escorez Cycloaliphatic hydrocarbon resinExxonmobil, 5300 Houston, TX TPO-L 2,4,6-trimehylbenzoylphenyl BASFCorporation, phosphinate Florham Park, NJ Heptane Heptane EMD MilliporeCorporation, Billerica, MA IPDI Isophorone diisocyanate/Desmodur BayerCorp., I Pittsburgh, PA, USA TMDI Mixture of 2,2,4-trimethyl-1,6-Evonik, Parsippany, diisocyanatohexane and 2,4,4- NJ, USAtrimethyl-1,6-diisocyanatohexane Silicone dimethylsiloxane dicarbinol,1732 Dow Corning, diol 1 MW/5562 CarbinolFluid Midland, MI Siliconedimethylsiloxane dicarbinol, 3650 Gelest diol 2 MW/DMS-C21 Morrisville,PA DMPA 2,2-dimethylol propionic acid/CAS Sigma-Aldrich Co., number4767-03-7 St. Louis, MO DBTDA Dibutyl tin diacetate/CAS numberSigma-Aldrich Co., 1067-33-0 St. Louis, MO BD 1,4-butanediolSigma-Aldrich Co., St. Louis, MO E1 Esacure One photoinitiator LambertiSPA, Oligo[2-hydroxy-2-methyl-1-[4-(1- Gallerate, Italymethylvinyl)phenyl]propanone EAD Epoxy Acrylate diol Denacol DA- NagaseAmerica, 920, polypropylene glycol (3) New York, NY diglycidyl etherreacted with acrylic acid PET Release Siliconature SILPHAN S50 onSiliconature USA, Liner 50 micron thick PET Chicago, IL TEAtriethylamine Sigma-Aldrich Co., St. Louis, MO PAX Polyaziridinecrosslinker PZ-28, Polyaziridine LLC, EW 167, CAS 64265-57-2 Medford, NJPreparation of Silicone Polyurethane 1

A 250 mL roundbottom was charged with 8.41 g IPDI (0.075644 eq), 40.80 gsilicone diol1 (0.047277 eq), 0.79 g DMPA (0.011819 eq), 12.74 g MEK,and 3 drops (˜0.06 g) DBTDA, placed in an oil bath and heated to 80 C.After 7 h at reaction temperature, the reaction was allowed to cool toRT overnight. The reaction was diluted with 58 g MEK and about half ofthe solution of 0.96 g BD (0.080371 eq) in 2 g MEK was added as thereaction was heated to 85 C internal temperature. After 30 min theremainder of the BD was added and rinsed in with about a gram of MEK.The reaction was monitored by FTIR and at the end of 7 h a small —NCOpeak was still noted at 2265 cm⁻¹. To the reaction was added 0.20 g of a10% by weight solution of BD in MEK solution, and reaction was continuedfor 3 h at 78 C. Some solvent was lost and the reaction was diluted to50% solids with MEK.

Preparation of Comparative Example 1

A 50 mL jar was charged with the materials indicated in Table 2 andshaken for a few seconds. The solution was coated using a 0.762 mmdrawdown bar onto a PET Release Liner and dried overnight at RT to anominal dried thickness of about 0.25 mm. The coating was placed in anoven at 80 C for 20 min. The coating was then laminated against itself.The resultant two thickness laminate was then cut in half and laminatedagainst itself again. The ˜1 mm laminate was used for further testing.

TABLE 2 Polyurethane PAX Percentage of Adhesive lA Triethylamine 20% inacid groups Assembly 50% in MEK 50% in MEK MEK reacted CE1 10 g 0.1170.121 12.5Preparation of Silicone Polyurethane 2

A 250 mL roundbottom was charged with 10.23 g TMDI (0.075644 eq), 52.53g silicone diol1 (0.060863 eq), 0.98 g DMPA (0.014607 eq), 16.25 g MEK,and 3 drops (˜0.06 g) DBTDA, placed in an oil bath and heated to 80 C.After 3 h at reaction temperature, an FTIR was taken of the reactionshowing a peak at 2265 cm⁻¹. The reaction at 3 h 10 min was diluted with48.75 g MEK and about half of the solution of 1.26 g BD (0.027997 eq) in2 g MEK was added. At 3 h 30 min the remainder of the BD was added andrinsed in with about a gram of MEK. The reaction was heated for 12 hmore at 80 C, and then monitored by FTIR showing no —NCO peak 2265 cm⁻¹.The reaction was adjusted to 50% solids with MEK.

Preparation of Examples 1 and 2 and Comparative Example 2

For each of the samples, a 50 mL jar was charged with the materialsindicated in Table 3 and shaken for a few seconds. The ˜1 mm laminateswere prepared as described in Preparation of Comparative Example 1.

TABLE 3 Percentage of Adhesive Polyurethane 2 Triethylamine PAX acidAssembly 50% in MEK 50% in MEK 10% in MEK groups reacted CE2 10 g 0.1170.121 12.5 Ex 1 10 g 0.117 0.242 25 Ex 2 10 g 0.117 0.484 50Preparation of Silicone Polyurethane 3

A 250 mL roundbottom was charged with 9.34 g TMDI (0.084028 eq), 40.79 gsilicone diol1 (0.047266 eq), 13.13 g Silicone diol 2 (0.007195 eq),0.98 g DMPA (0.014607 eq), 16.25 g MEK, and 3 drops (˜0.06 g) DBTDA,placed in an oil bath and heated to 80 C. After 3 h at reactiontemperature, an FTIR was taken of the reaction showing a peak at 2265cm⁻¹. The reaction at 3 h 10 min was diluted with 48.75 g MEK and abouthalf of the solution of 0.76 g BD (0.016911 eq) in 2 g MEK was added. At3 h 10 min the remainder of the BD was added and rinsed in with about agram of MEK. The reaction was heated for 16 h more at 80 C, and thenmonitored by FTIR showing a small —NCO peak 2265 cm⁻¹. To the reactionwas added 0.45 g of a 10% by weight solution of BD in MEK, 0.045 gsolids (0.001 eq). After 4 h after the 0.045 g BD addition, a very small—NCO peak remained at 2265 cm⁻¹, and 0.28 g of a 10% by weight solutionof BD in MEK, 0.028 g solids (0.00062 eq) was added. After 2 h the —NCOpeak at 2265 cm⁻¹ was insignificant. The reaction was adjusted to 50%solids with MEK.

Preparation of Examples 3-5

For each of the samples, a 50 mL jar was charged with the materialsindicated in Table 4 and shaken for a few seconds. The ˜1 mm laminateswere prepared as described in Preparation of Comparative Example 1.

TABLE 4 Percentage of Adhesive Polyurethane 3 Triethylamine PAX acidAssembly 50% in MEK 50% in MEK 10% in MEK groups reacted Ex 3 10 g 0.1130.59 6.25 Ex 4 10 g 0.113 0.117 12.5 Ex 5 10 g 0.113 0.234 25Preparation of Silicone Polyurethane 4

A 250 mL roundbottom was charged with 10.69 g IPDI (0.096146 eq), 0.78 gEAD (0.00361 eq), 16.25 g MEK, and 3 drops (˜0.06 g) DBTDA, placed in anoil bath and heated to 75 C for 30 min. Next 51.86 g silicone diol 1(0.06009 eq), and 0.97 g DMPA (0.01442 eq) were added and reacted for 3h. The reaction was then analyzed by FTIR, showing a small —NCO peak at2265 cm⁻¹. To the reaction was added 48.75 g MEK followed by 0.71 g BD(0.015744). The reaction was allowed to proceed for 12 h at 75 C. At theend of this time the reaction showed no —NCO peak. The solids wereadjusted to 50% solids with MEK.

Preparation of Silicone Polyurethane 5

In a manner similar to that of the Preparation of Polyurethane 4, 9.25 gIPDI (0.08323 eq), 0.78 g EAD (0.00361 eq), 3 drops (˜0.06 g) DBTDA in16.25 g MEK were reacted for 30 min at 75 C followed by addition of40.40 g silicone diol 1 (0.046817 eq), 13.00 g silicone diol 2 (0.07127eq), and 0.97 g DMPA (0.0144 eq) and reaction for 3 h. The reaction wasthen diluted with 48.75 g MEK and 0.59 g BD (0.01314 eq) was addedfollowed by reaction for 12 h, and adjustment of solids to 50% with MEKto provide Silicone Polyurethane 5.

Preparation of Example 6 and Comparative Example 3

To Polyurethane 4, 10.0 g solution at 50% solids, was added 0.50 g of a10% solution of Esacure KB1. Similarly, to Polyurethane 5, 10.0 gsolution at 50% solids, was added 0.50 g of a 10% solution of EsacureOne. Each solution was coated as described in Preparation of ComparativeExample 1. The coatings were dried overnight at RT, then in an oven for5 min at 80 C. Each coating was then cured using a nitrogen purgedFusion Systems device with a 300 watt Fusion D bulb (Fusion Systems,Inc., Gaithersburg, Md.) at a conveyer speed of 9.14 m/min. From thispoint, the ˜1 mm laminates were prepared as described in Preparation ofComparative Example 1. See Table 5.

TABLE 5 Adhesive Assembly Polyurethane CE3 4 Ex 6 5Examples 7 and 8: Preparation of Polyisobutylene-Based Assembly LayerSamples

Assembly layer films were prepared according the compositions providedin Table 6. In Example 7, 5.876 g of B 50 SF, 0.293 g of SR 833 S, 0.029g of TPO-L, and 12.011 g of heptane were added in a glass vial. The vialwas sealed and contents were mixed overnight. The solution was thencoated on a 50 μm thick siliconized polyester release liner, RF02N (SKCHaas, Korea) using a knife coater with a gap of 8 mil. The coated samplewas placed in an oven at 70° C. for 15 minutes. This coated sample wasthen irradiated with an H-bulb with a total dose of 1200 mJ/cm² of UV-A.In Example 8, 8.333 g of B 50 SF, 0.416 g of Escorez 5300, and 16.667 gof heptane were added in a glass vial. The vial was sealed and contentswere mixed overnight. The solution was then coated on a 50 μm thicksiliconized polyester release liner, RF02N (SKC Haas, Korea) using aknife coater with a gap of 8 mil. The coated sample was placed in anoven at 70° C. for 15 minutes. In comparative example 4 (CE4), 6 g and12.011 g of heptane were added in a glass vial. The vial was sealed andcontents were mixed overnight. The solution was then coated on a 50 μmthick siliconized polyester release liner, RF02N (SKC Haas, Korea) usinga knife coater with a gap of 8 mil. The coated sample was placed in anoven at 70° C. for 15 minutes.

TABLE 6 Preparation of Polyisobutylene-based assembly layer samples B 50Escorez SF SR833 5300 TPO- Example (g) S (g) (g) L (g) Heptane CE4 612.011 Ex 7 5.876 0.293 0.029 11.733 Ex 8 8.333 0.416 16.667Examples 7-8 and Comparative Examples CE4 were tested for T_(g), shearcreep, shear modulus and shear stress as described in Test Methods 2-3described above. Data are recorded in Table 7 below.

TABLE 7 Rheological data Shear Creep Strain @ Recov- Shear Modulus G′ 95kPa ery @ Exam- T_(g) 40° C. 20° C. 0° C. −20° C. Stress 60 s ple ° C.kPa kPa kPa kPa % % CE1 NT 21.9 93.3 473.7 2440.9 732 71.5 CE2 NT 4.59.7 29.4 114.4 3737 8.5 CE3 NT 34.8 138.8 593.1 2447.6 440 81.4 Ex1 NT10.1 15.8 38.0 117.4 791 94.8 Ex2 NT 23.8 30.9 62.4 151.7 350 95.9 Ex3NT 21.6 93.4 383.1 1516.1 907 63.5 Ex4 NT 27.9 96.2 363.0 1296.0 64983.8 Ex5 NT 39.3 108.4 363.9 1187.3 316 96.8 Ex6 NT 15.7 69.2 302.51206.8 723 79.2 CE4 −42 246.2 272.1 301.9 485.4 848 43.2 Ex7 −41 240.7285.3 340.7 583.6 189 90.3 Ex8 −40 218.1 246.5 286.9 528.9 257 71.8 NT:Not Tested

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. An assembly layer for a flexible devicecomprising: an olefin-based adhesive composition, wherein within atemperature range of between about −30° C. to about 90° C., the assemblylayer has a shear storage modulus at a frequency of 1 Hz that does notexceed about 2 MPa, a shear creep compliance (J) of at least about6×10⁻⁶ 1/Pa measured at 5 seconds with an applied shear stress betweenabout 50 kPa and about 500 kPa, and a strain recovery of at least about50% at at least one point of applied shear stress within the range ofabout 5 kPa to about 500 kPa within about 1 minute after removing theapplied shear stress, wherein the olefin-based adhesive compositioncomprises at least one of a polyisoprene, a polybutadiene, an olefinblock copolymer, a polyisobutylene, and high alkyl polyolefin, andwherein the assembly layer is optically clear.
 2. The assembly layer ofclaim 1, wherein the flexible device is an electronic display device. 3.The assembly layer of claim 1, wherein the assembly layer has a glasstransition temperature of up to about 10° C.
 4. A laminate comprising: afirst flexible substrate; a second flexible substrate; and an assemblylayer positioned between and in contact with the first flexiblesubstrate and the second flexible substrate, wherein within atemperature range of between about −30° C. to about 90° C., the assemblylayer has a shear storage modulus at a frequency of 1 Hz that does notexceed about 2 MPa, a shear creep compliance (J) of at least about6×10⁻⁶ 1/Pa measured at 5 seconds with an applied shear stress betweenabout 50 kPa and about 500 kPa, and a strain recovery of at least about50% at at least one point of applied shear stress within the range ofabout 5 kPa to about 500 kPa within about 1 minute after removing theapplied shear stress, wherein the assembly layer comprises anolefin-based adhesive composition comprising at least one of apolyisoprene, a polybutadiene, an olefin block copolymer, apolyisobutylene, and high alkyl polyolefin, and wherein the assemblylayer is optically clear.
 5. The laminate of claim 4, wherein at leastone of the first and second substrates is optically clear.
 6. Thelaminate of claim 4, wherein the laminate has a haze value of less thanabout 5% after the laminate is placed in an environment of 70° C./90%relative humidity for 72 hours and then cooled to room temperature. 7.The laminate of claim 4, wherein the assembly layer has a glasstransition temperature of up to about 10° C.
 8. The laminate of claim 4,wherein the laminate does not exhibit failure when placed within achannel forcing a radius of curvature of less than about 15 mm over aperiod of 24 hours at room temperature.
 9. The laminate of claim 8,wherein the laminate returns to an included angle of at least about 130degrees after removal from the channel after the 24 hour period at roomtemperature.
 10. The laminate of claim 4, wherein the laminate does notexhibit failure when subjected to a dynamic folding test at roomtemperature of about 10,000 cycles of folding with a radius of curvatureof less than about 15 mm.
 11. A method of adhering a first substrate anda second substrate, wherein both of the first and the second substratesare flexible, the method comprising: positioning an assembly layerbetween the first flexible substrate and the second flexible substrateto form a laminate, wherein within a temperature range of between about−30° C. to about 90° C., the assembly layer has a shear storage modulusat a frequency of 1 Hz that does not exceed about 2 MPa, a shear creepcompliance (J) of at least about 6×10⁻⁶ 1/Pa measured at 5 seconds withan applied shear stress between about 50 kPa and about 500 kPa, and astrain recovery of at least about 50% at at least one point of appliedshear stress within the range of about 5 kPa to about 500 kPa withinabout 1 minute after removing the applied shear stress, wherein theassembly layer comprises an olefin-based adhesive composition comprisingat least one of a polyisoprene, a polybutadiene, an olefin blockcopolymer, a polyisobutylene, and high alkyl polyolefin, and wherein theassembly layer is optically clear; and applying at least one of pressureand heat to form a laminate.
 12. The method of claim 11, wherein thelaminate does not exhibit failure when placed within a channel forcing aradius of curvature of less than about 15 mm over a period of 24 hoursat room temperature.
 13. The method of claim 12, wherein the laminatereturns to an included angle of at least about 130 degrees after removalfrom the channel after the 24 hour period at room temperature.
 14. Themethod of claim 11, wherein the laminate does not exhibit failure whensubjected to a dynamic folding test at room temperature of greater thanabout 10,000 cycles of folding with a radius of curvature of less thanabout 15 mm.
 15. The assembly layer of claim 1, wherein the olefin-basedadhesive composition is substantially free of silicone.
 16. The assemblylayer of claim 1, wherein the olefin-based adhesive compositioncomprises at least about 50% of an olefin polymer.
 17. The assemblylayer of claim 1, wherein a majority component of the assembly layercomprises an olefin polymer.
 18. The assembly layer of claim 1, whereina highest weight component of the assembly layer comprises an olefinpolymer.